Modern data storage systems use servopositioning (or, “servo”) systems to guide their recording and playback components with respect to a recording medium, and thus enable high track density, which increases data storage capacity. Errors in the ability to follow the servopositioning signals on the medium can cause unacceptable reductions in storage capacity, recording/playback rates, and other parameters that are important to consumers (and thus to system manufacturers).
Traditionally, there have been two distinct styles of servo patterns or formats for linear magnetic tape recording systems. One type employs so-called time-based servo techniques, an example of which is disclosed in U.S. Pat. No. 5,689,384. Commercial magnetic tape drives such as the IBM model 3570 and drives known under the names “Ultrium” and “Accelis” as described by the Linear Tape Open consortium, use time-based servopositioning systems. Another type employs so-called amplitude-based servo techniques, examples of which are disclosed in U.S. Pat. Nos. 5,426,543 and 5,898,533.
Each style of servo technique has advantages and disadvantages. The advantages of time-based servo systems include very wide dynamic range; inherent track identification; low DC centerline error; and the ability to qualify position error signal (PES) validity by the amplitude of the servo signal. Disadvantages include extreme sensitivity to tape speed during writing; sensitivity to high frequency speed error during reading; and poor scalability to very small track pitches. The advantages of amplitude-based servo systems roughly mirror the disadvantages of time-based servo systems, i.e., they include insensitivity to tape speed during writing; insensitivity to tape speed error during reading; and good scalability to very small track pitches. Similarly, the disadvantages of amplitude-based servo systems roughly mirror the advantages of time-based servo systems, i.e., they include limited dynamic range, no inherent track identification, considerable DC centerline error, and no secondary PES validity check.